The present invention relates to the field of assembling circuit boards. Specifically, the present invention relates to a method and system for connecting a chip package to its land grid array contacts on a circuit board by applying a pre-loading force to compression components.
With the ever increasing complexity of components such as microprocessors and application specific integrated circuits (ASICs) comes greater challenges in forming good electrical connections between the component and a printed circuit board. For example, these components may have hundreds of xe2x80x98pinsxe2x80x99 to be connected to the printed circuit board. Land grid array sites are a popular way to connect such components to a printed circuit board. A land grid array may comprise an array of contact pads on the component that are merged with similar contact pads on the printed circuit board. An interposer between the chip package and the printed circuit board provides a frame that supports the chip package and also provides a conductive path for each of the contact pads.
In order to form a good electrical contact with such land grid array assemblies, the interposer""s conductors need to be compressed. Thus, a normal force is applied to compress together the chip package and printed circuit board with the interposer sandwiched between. This force must be uniform, otherwise some of the contact pads will compress more than others, which may lead to a poor overall electrical contact. To help provide a uniform compression, land grid array sites are conventionally constructed using springs to provide a coupling force loading to a loading plate or a heatsink/heatpipe assembly. This ultimately ensures good electrical connection of a chip package to its land grid array contacts on a printed circuit board. However, conventional methods may require complex fastener torquing sequences to compress the springs, may compress the contact pads unevenly, and may apply too great a load.
FIG. 1 illustrates one method for forming an electrical contact between a component and a printed circuit board using a land grid array. However, this conventional method requires that a complex sequence of torques be applied to fasteners (e.g., screws or nuts 135) to properly attach the component such that a good electrical contact is formed between the printed circuit board 110 and the chip package (e.g., a processor, ASIC, etc.). Unfortunately, such torque sequences are not always reliable.
FIG. 1 shows a printed circuit board 110 resting in a supporting fixture 120. A heat sink 130 is being attached via four fasteners. The fastener may comprise a threaded fixture (not visible) coming up from the printed circuit board 110 with a nut 135 threaded on top. A compression component 140 (e.g., a spring) around the threaded fixture and between the bottom of the nut 135 and the heat sink 130 applies a coupling force to the heat sink 130 in response to the nut 135 being threaded down the threaded fixture. A conductive interposer (not visible) resides between the printed circuit board 110 and a chip package (not visible) underneath the heat sink 130. Thus, the chip package, interposer, and printed circuit board 110 are compressed together. However, this compression should be uniform and of proper magnitude to form good electrical contacts and not damage any components.
Thus, in this conventional method, the nuts 135 may not be simply tightened one at a time because that would result in an uneven load on the heat sink 130, interposer, etc., leading to a poor electrical contact. Thus, conventionally a small torque is applied to one nut 135, which compresses its spring 140 slightly, which in turn slightly increases the load to the heat sink 130 and components below the heat sink 130. However, this technique cannot apply to large a torque to the nut 135 or else the spring 140 would compress too much and apply too large a load on the heat sink 130 relative to the load on the heat sink 130 from the other springs 140. In other words, an uneven load on the heat sink 130 would result. Thus, the problems just mentioned will arise.
Continuing on with this conventional process, after the small torque is applied to the first nut 135, a small torque is applied to a second nut 135 to cause a small load to the heat sink 130 though the spring 140 around a second threaded fixture. Typically, all four nuts 135 receive this first small torque before applying a second small torque to each nut 135. Eventually, all the nuts 135 are tightened. However, the process can be relatively time consuming as a torque driver 150 may have to be repeatedly moved and re-positioned on the next nut 135. Furthermore, each nut 135 may only be threaded partially down the threaded fixture each step. Furthermore, this conventional technique must either count the number of turns or the torque applied to the nuts 135. Counting turns provides only a rough estimate of the torque applied and is hence inaccurate. Measuring the torque each time requires that the torque driver 150 be properly calibrated to measure fine torques. Furthermore, each time the sequence of tightening the nuts 135 is repeated, a new, higher, torque value is required. Thus, either the torque driver 150 must be adjusted or a different torque driver 150 must be used. This results in a time consuming and extremely cumbersome process to complete the assembly of the printed circuit board.
An advancement over the prior conventional method is to apply a pre-load force directly to a component, as seen in FIG. 2. Thus, the force may be applied directly onto the chip package. Alternatively, the force may be applied onto a heat sink/pipe or loading plate above the chip package. Unfortunately, this method causes loads that are greater than the final (e.g., working) load and creates unbalanced loads.
Referring now to FIG. 2, the press ram 210 compresses the heat sink 130. However, when the nuts 135 are torqued down, the heat sink 130 is subjected to a cumulative load from both the press ram 210 and the springs 140. Thus, the load is much higher than the final working load (e.g., twice the working load). This subjects the interposer contacts to higher loads than intended and it may not form a good electrical contact with the printed circuit board 110. Furthermore, the method shown in FIG. 2 may also load the heat sink 130 (and other components) unevenly unless care is taken to carefully step through a sequence in which the tightening of the nuts 135 proceeds with a sequence of stepwise tightening.
Additionally, the extra load may damage the circuit board 110 and associated components, resulting in a defect. Unfortunately, this defect may not be discovered until much later in the manufacturing process. Thus, considerable extra expense may go into assembling the circuit board 110 before the defect is detected. Furthermore, the defect may not be detected at all, and thus the customer receives a part with a latent defect.
Thus, one problem with conventional methods and systems for connecting components to a circuit board using a land grid array is that they may require application of complicated torque sequences. Another problem with such conventional methods and systems is that they may overload or apply an uneven load during the process. Thus, conventional methods and systems may damage the printed circuit board and/or its components in the connection process and/or result in an inadequate electrical connection between a chip package and the printed circuit board.
The present invention pertains to a device for assembling circuit boards. The device comprises an upper surface for receiving a compressing force. The device also has a lower surface for compressing a number of compression devices in a land grid array assembly while allowing access to a number of fasteners associated with the compression devices. The device is able to assist in the formation of an electrical contact between a chip package in the land grid array assembly and a circuit board by the lower surface being pressed against the compression devices to compress the compression devices and then allowing the plurality of fasteners to be tightened.